1. Field of the Invention
The invention concerns a fastening device for connecting a valve spring retainer to a valve stem according to the precharacterizing clause of claim 1.
2. Description of the Related Art
The printed specification DE 42 07 213 A1 discloses a connection between the stem of a valve of an internal combustion engine and a valve spring retainer, which forms a support for a valve spring. The valve spring retainer, held on the valve stem, is secured against axial displacement on the valve stem by means of a sleeve-shaped clamping cone with a conical outer contour, in that the clamping cone is pushed onto the valve stem and inserted into a cylindrical, concentric receptacle of the spring retainer. The clamping cone is arranged on the side of the connection of the valve spring retainer and valve stem that is facing away from the valve spring; both the clamping cone and the valve spring retainer are held axially immovably on the valve stem in the clamping position of the clamping cone.
This fastening has the disadvantage that it is only possible with difficulty for the clamping cone to be released from the clamping position, because the conical outer contour of the clamping cone is seated in the receptacle of the spring retainer with high clamping force and there is the risk of material corrosion. Adjustment of the valve spring retainer, which may be required for example as a result of wear and is to be carried out after assembly, is made more difficult by this.
The invention is based on the problem of providing a fastening device between a valve spring retainer and a valve stem of an internal combustion engine that is functionally dependable over a long operating period and is easy to assemble and disassemble. The fastening device is in particular to be capable of being positioned with little expenditure of force steplessly on the valve stem. The fastening device is expediently to be capable of being loaded in both axial directions. The present invention is directed towards one or more of the problems set forth above.
The clamping sleeve which secures the valve spring retainer on the valve stem in a captive manner is held on the valve stem exclusively with non-positive engagement. This makes it possible to form the valve stem with an uninterrupted, continuous circumferential surface without grooves or projections, whereby on the one hand any weakening of the valve stem is avoided, so that the valve stem can in particular transfer higher axial forces, and on the other hand simpler and more cost-effective production of the valve stem is made possible. In addition, the assembly of the fastening device is simplified. Furthermore, the fastening device is of a small size.
The valve spring retainer can be adjusted steplessly on the valve stem and can be fastened on the valve stem in any axial position, whereby an exact positioning of the valve stem including the valve retainer is possible. The fastening device according to the invention is suitable in particular for electromagnetic valve controls in which an electromagnetic actuating means, acting on the valve stem, has to be adjusted in a mid-position between two magnets and it is necessary to compensate for any play, thermal expansions, wear etc. occurring.
A further advantage is that the advantageous material properties of the shape memory alloy can be used for establishing a secure connection. Shape memory alloys are distinguished in that their mechanical properties can be changed over a wide range by the influence of temperature or external stress. This effect can be used to provide a fastening with high clamping force which nevertheless can be released again at any time. Both the effect of the so-called suppressed shape memory and the effect of the superelasticity of the alloy can be utilized in particular. Nickel-titanium alloys, if appropriate also nickel-titanium-copper alloys, are used with preference. However, copper- or iron-based shape memory alloys may also be used.
In the case of the suppressed shape memory, the clamping sleeve, the internal diameter of which is smaller than the external diameter of the valve stem, is expanded plastically by mechanical action at a temperature at which a martensitic structure of a relatively soft consistency is formed, until unproblematical, essentially force-free pushing of the clamping sleeve over the valve stem is possible. Subsequently, a temperature increase above the switching temperature takes place, whereupon the clamping sleeve contracts due to transformation into an austenitic structure and has the tendency to revert to its original shape. For reasons of functional dependability, the transformation back from austenite to martensite should commence only at low temperatures, in order that the restoring force in the clamping sleeve in the form of a high radial stress is reliably available, to allow the valve spring retainer to be secured on the stem of the valve. This temperature requirement is met by cryogenic shape memory alloys. For releasing the connection, the clamping sleeve is cooled with liquid nitrogen or with other refrigerants, whereupon the softer martensitic structure forms again, the radial stress subsides and the clamping sleeve can be pulled off.
Shape memory alloys with an extended hysteresis may also be used. These have at room temperature a martensitic structure which is soft and easily deformable and can be expanded at room temperature. The clamping sleeve is pushed onto the valve stem at room temperature, in the expanded state, free from friction and forces and is subsequently heated to 165xc2x0 C., whereby the structure is transformed into hard austenite. The clamping sleeve tries to revert to its original dimensions and thus builds up a radial stress, which holds the clamping sleeve on the valve stem with non-positive engagement. This stress only subsides again on transformation back from austenite to martensite, which in the case of shape memory alloys with an extended hysteresis does not take place until a temperature as low as xe2x88x92120 C.
Instead of the effect of the suppressed shape memory, the effect of the superelasticity may also be utilized. The superelasticity is distinguished by an elastic material behaviour up to approximately 8% elongation without any change in temperature. Expediently used as the material is likewise a shape memory alloy that is austenitic at room temperature, which is expanded for pushing onto the valve stem. As in the case of the suppressed shape memory, this connection is also released by cooling the clamping sleeve below the temperature at which the austenite has transformed completely into martensite.
In an advantageous development, the fastening device comprises a conical clamping part which is placed onto the valve stem and onto which the valve spring retainer is pushed. The clamping sleeve is pushed onto the stem on the side of the valve spring retainer that lies opposite from the valve spring, so that the valve spring presses the spring retainer against the clamping part and the clamping part is axially blocked by means of the clamping sleeve. In this configuration, it is sufficient to form the clamping sleeve as a simple clamping ring which is arranged on the axially outer end of the valve stem.
It may be expedient to form the clamping part and the clamping sleeve as a one-part, integral component from a shape memory alloy. This reduces the number of different parts and simplifies assembly and disassembly. In a development of the invention, the clamping sleeve is captively fastened on the valve spring retainer, in particular by welding, soldering etc. In addition, it may be appropriate to produce the entire valve spring retainer, including the clamping sleeve formed on it, from a shape memory alloy.